Due to the progress in the semiconductor technology, integrated circuit (IC) chips have been widely used in various electronic apparatus, such as personal computers, notebook computers and network servers. While the IC chips have significantly increased computing speed and functions, they also generate correspondingly increased waste heat. Such waste heat must be effectively removed to protect the electronic apparatus against failure. Various heat dissipation means are therefore developed to achieve effective removal of the heat generated by the IC chips.
One of the heat dissipation means is loop heat pipe (LHP). In a conventional loop heat pipe structure, there is included a reservoir or a compensation chamber for storing an adequate amount of working fluid, so that the evaporator for the loop heat pipe structure can be properly furnished with the working fluid and adapt to the volume change of the working fluid caused by density change thereof. The reservoir or compensation chamber also filters gas or bubbles in the working fluid, so that the working fluid is not interfered and damaged by the containing gas or bubbles.
While the conventional loop heat pipe structure provides a lot of advantages, it has a cylindrical evaporator that occupies a relatively large space and fails to directly contact with the heat source due to the round outer surface thereof. To overcome such disadvantages, a flat plate loop heat pipe (FPLHP) structure has been developed. The existing flat plate loop heat pipe structure has only type of wick structure provided in the evaporator thereof. With only one type of wick structure provided in the evaporator, heat that heats the evaporator can also easily enter into the reservoir or compensation chamber to cause serious heat leak. In the event a single layer of wick structure with low thermal conductivity is used to prevent the heat leak, another problem of relatively high local thermal resistance would occur in the evaporator. On the other hand, when a single layer of wick structure with high thermal conductivity is used, it would become very difficult to initialize the flat plate loop heat pipe structure. In other words, the flat plate loop heat pipe structure having one single layer of wick structure with high thermal conductivity requires an extremely high critical power for the initialization thereof and even could not be started up in some special working condition.
Moreover, the existing flat plate loop heat pipe structure usually has an evaporator made of only one type of material for both of its wall portions and bottom. However, the bottom of the evaporator in contact with the heat source should have higher thermal conductivity than the wall portions of the evaporation. Further, due to the special construction of the flat plate loop heat pipe structure, when the bottom of the evaporator is in contact with the heat source, the heat is also transferred via the wall portions of the evaporator to heat the working fluid in the reservoir or compensation chamber. In some cases, the amount of heat transferred to the reservoir or compensation chamber is even equal to that causing the heat leak via the wick structure in the evaporator. A combined effect of the above two conditions badly affects the thermal performance of the flat plate loop heat pipe structure to even offset the advantages thereof.